The present invention relates to a surgical laser system and, more particularly, to a solid-state laser system characterized by high average power and by near- to mid infra-red range of wavelengths.
A widely used class of surgical laser systems that operate in the near IR range is characterized by a solid-state laser and an articulated-arm beam transmission means. Solid-state lasers are described and explained in many publications--for example in "Solid State Laser Engineering" by Walter Koechner (2nd edition), Springer Publishing Co., 1988, which is incorporated by reference for all purposes as if fully set forth herein. Articulated-arm beam transmission is well-known in the art.
In this class of surgical laser systems the laser medium usually consists of Yttrium-Aluminum-Garnet (YAG), in which are imbedded rare earth dopant ions--typically Neodymium (Nd), Erbium (Er) or Holmium (Ho), respectively producing wavelengths of approximately 1, 3 or 2 microns. A typical articulated-arm beam transmission means (to be referred to, for brevity, as an arm) is about two meters long, includes seven mirrors to deflect the laser beam past the articulations, and has an internal diameter of 15 to 18 mm.
Input (or pumping-) power is usually supplied to the laser medium by radiation from an external light source, such as an arc lamp (in which case the input power and the laser beam are continuous) or a flash lamp (in which case the laser operates in pulses).
The optical efficiency of the laser medium, that is, the ratio of laser power generated in the optical cavity to the light power fed into the laser medium, is relatively low--generally in the range of 0.1-5% and typically 1%. The excess input light power is converted to heat and acts to raise the temperature of the laser medium. Thus, for example, for a laser beam power level of 5 watts, the thermally absorbed power level is typically 5/0.01=500 watts. In order to keep the temperature at acceptable levels, the generated heat is conducted away from the laser medium rod--typically by means of a jacket of flowing water surrounding the rod. In the laser medium rod, heat is absorbed from the pumping light about evenly over any cross section, while heat loss to the cooling water takes place over the outer surface of the rod. Since the thermal conductivity of the rod material is low, this causes a very pronounced temperature gradient radially across the rod, which in turn causes differential elongation of the rod--the region near its axis being longer than the peripheral region--resulting in convex end surfaces of the rod, which thus function as positive lenses. This phenomenon is called "thermal lensing".
Because of this thermal lensing, the optical cavity mirrors must be placed as close as possible to the laser rod in order to maintain a stable optical resonator. The resulting relatively short optical cavity has a high Fresnel number, and therefore supports a high number of transverse modes. As a result, the beam of coherent light that emerges is highly divergent. The beam diameter at the tool end of the articulated arm therefore becomes large, possibly exceeding the internal aperture and causing vignetting of part of the beam and loss of power delivered to the target. The following numerical illustration is offered as a typical example: The exit beam is 6 mm in diameter and diverges at an angle of 20 milliradians; at the end of a 2-meter-long arm, the beam diameter becomes 6+0.02*2000=46 mm, which greatly exceeds the maximum internal diameter of a practical arm, which, as noted above, is about 18 mm.
In addition, when the laser is turned on, before the rod reaches thermal equilibrium, it may focus the beam on the optical cavity mirrors, thereby damaging those mirrors.
The effects described hereabove limit the practical operating power level of surgical laser systems of prior art, resulting in a maximum beam power of about 5 to 10 watts at the exit aperture of the articulated arm.
In surgical laser systems of prior art, alleviation of the problem of beam divergence has been attempted by simply shortening the length of the articulated arm. This has a severe disadvantage in that it limits the range of tool motion and of patient placement available to the surgeon. Alternatively widening the diameter of the last sections of the arm makes them heavy and more cumbersome. Another way suggested to alleviate the problem is to place a mode filter in the optical cavity, thus reducing the resultant beam divergence; this will, however, further reduce the efficiency of the laser.
One obvious solution of the problem of thermal lensing is to introduce a strongly negative lens into the optical cavity near each end of the rod, such that will nullify the positive thermal lens effect. This will, however, work only at one particular power level. In contrast, surgical laser systems are typically operable at widely varying power levels. If the negative lens is strong enough to nullify the effect of the thermal lens at maximum power level, then at lower power levels the optical divergence introduced by the negative lens will tend to decrease the efficiency of the laser and to raise the power input threshold at which lasing will occur at all.
There is thus a widely recognized need for, and it would be highly advantageous to have, a surgical solid-state based laser system that delivers through an easily manipulatable articulated arm a beam of 2-3 microns wavelength at varying power levels reaching well above those hitherto achievable, without damaging internal parts.